1. Field of the Invention
The invention also concerns an apparatus for implementing said method.
2. Description of the Prior Art
Cleaning of coater backing rolls and calender rolls, particularly those of soft calenders has been necessary as long as such equipment has been in use. Dirt accumulates on the backing rolls as the coating mix penetrates the paper web during coating via the web pores and adheres onto the roll surface. The roll may also gather dirt as the coating mix finds access via holes in the base paper. Rolls of soft calenders accumulate adhering matter from dirt and coat dust carried over by the moving web and from areas of thick coats that have remained moist thus permitting their adherence on the roll surface. The penetration depth of the coating mix, and thus, its filtration through the web, is determined by the paper web porosity and thickness as well as the process conditions such as the application/doctor blade pressure and coating mix properties. With the modern trends of using a thin base web and a high coat weight, the coating mix is filtered in greater amounts through the base web thus resulting in heavier dirt accumulation on the rolls than encountered before. A coat clump adhered to the roll becomes firmly fixed to the roll through drying and then causes marking on the paper sheet. Paper quality degradation through marking obviously increases as the accumulation of dirt on the roll progresses.
Cleaning the rolls has in the prior art been performed by means of manual washing and doctoring with a doctor blade. Manual washing with a wetted sponge is clumsy and time-consuming and requires heavy manual labor. Due to high web speeds and greater tendency to dirt accumulation, the need for cleaning the rolls is frequent, since the rolls become dirty so rapidly. As the manual cleaning of the rolls is slow and accumulation of dirt is fast, manpower must be allocated on an almost continuous basis for the cleaning job. The work must be performed carefully, because no water drippage onto the running web is allowed and the operation must be carried out while the equipment is running. As modern equipment is designed for high web speeds and wide webs, manual cleaning of long rolls is extremely awkward and dangerous.
Manual washing of rolls can be replaced by the use of doctor blades. Such an arrangement performs dirt removal from roll surface with the help of a doctor blade. Water is used to aid cleaning. Drawbacks of cleaning by doctoring include splashing, wear of the roll surface by the doctor blade and poor function at high web speeds exceeding 10 m/s (600 m/min). Splashing results in an untidy working environment, and when water lands on the web, it spoils the paper sheet being processed. Owing to alignment tolerances between the roll to be cleaned and the doctor blade, the doctor blade imposes an uneven linear load on the roll causing a high wear rate of both the blade and the roll. Due to the extremely stringent requirements set on the roll straightness, even a slightest wear of the roll leads to impairment of paper sheet quality thus necessitating roll replacement. The change of the expensive roll combined with the mandatory equipment shutdown adds up to higher operating costs.
In the light of the above-discussed, a modem roll cleaning method of higher practical usefulness is obviously needed.